At the point at which an antenna receives it, a radio frequency (RF) signal is not only typically far too weak to be processed, but also contains unwanted “blockers.” Blockers are artifacts that superpose and interfere with the received signal and include noise from external sources and perhaps near-end crosstalk (NEXT) from simultaneous transmissions from the transceiver itself. The receive chain of the transceiver is therefore provided with an LNA and a resonant circuit to reduce the artifacts and amplify the received signal before processing occurs.
The goal of a well-designed LNA and resonant circuit is to amplify the received signal and reject the blockers. This is known as “discrimination.” Effective discrimination is beneficial for transceivers of all kinds, including Global System for Mobile communication (GSM) compliant transceivers. However, it is essential for transceivers that transmit and receive concurrently, such as Universal Mobile Telecommunications System (UMTS) compliant transceivers. Unfortunately, achieving effective discrimination in the latter context is particularly challenging, since the blockers potentially include both noise and NEXT.
If the LNA and the resonant circuit provide effective discrimination, performance requirements for additional circuitry downstream in the receive chain (e.g., the mixer) can be relaxed. For example, the linearity, or dynamic range, requirement of the mixer can be reduced without compromising the transceiver's operation. As a result, the downstream circuitry can be less expensive, lowering the overall cost of the transceiver.
The resonant circuit takes the form of an inductance-capacitance (LC) circuit in which the inductance is in parallel with the capacitance and is tuned to resonate at a particular frequency, e.g., the center frequency of a particular channel. Unfortunately, manufacturing variations and defects often result in LC circuits that do not resonate at their intended frequencies. Even small variations in resonant frequency can move the resonant circuit off-channel and drastically degrade discrimination.
The capacitance values of integrated circuit (IC) based LC circuits do not vary continuously; instead, they vary by quantized (digital) amounts. The least-significant bit (LSB) of the digital number representing the capacitance value (known as a “capacitor code”) thus is the smallest capacitance value by which an LC circuit can be changed. For example, a change in the capacitance LSB can move the resonant frequency of an LC circuit off the target center frequency by 2-5 MHz, depending on the center frequency value and the resolution of the capacitance LSB. Extremes in manufacturing process variations, sometimes called “process corners,” may result in a capacitor code several codes away from a required capacitor code. An error of more than 2-3 codes can result in unacceptable discrimination and require, for example, a higher linearity range in the mixer.
Accordingly, what is needed in the art is a device and method that improves receive chain discrimination.